Marine seismic exploration methods uses a seismic source which transmits a seismic signal, while a receiving device measures the amplitudes and arrival times of the seismic signals returned (reflected/refracted) by discontinuities in the sub surface. The discontinuities are formed by interfaces between layers having different elastic properties and are called seismic reflectors. The returned seismic signals are recorded by seismic sensors at the ocean bottom or near the sea level.
In marine seismic exploration two main techniques are used to record the returned seismic signals. One is by using hydrophone cables that are towed behind a vessel. This technique only records the pressure waves (P-waves) since the shear waves (S-waves) do not propagate through the water column. The other technique is to deploy seismic nodes that contain both hydrophones and geophones at the ocean bottom. By doing so both P-waves and S-waves can be recorded and hence more useful data will be recorded and subsequently processed and used for mapping the sub surface.
During recent years, there has been an increased effort to improve the results of marine seismic investigations by collecting seismic signals at the ocean bottom instead of using the more usual vessel towed hydrophones for signal recording.
We will in the following describe the existing, known methods for acquisition of marine seismic data using seismic sensors located on the ocean bottom. There are basically two principal methods that are used at present for collecting seismic data using seismic sensors.
The first method is to deploy an ocean bottom cable with integrated seismic sensors and electrical and/or optical wiring from the seismic sensors to the sea level where the seismic data is recorded. The seismic signals are generated by a seismic source deployed and towed by a source vessel. During data recording the cable is normally attached to a recording vessel or the cable deploying vessel. In the last couple of years a slightly different approach has been in use whereby the separate cable deploying vessel has been replaced with a recording buoy that also provides the cable with electrical power generated from either a diesel generator or from batteries located in the buoy. All or part of the recorded data is then transmitted via a radio link from the buoy to either the source vessel or the cable deploying vessel.
The second present method that is used is to deploy and recover autonomous seismic data recording nodes to and from the ocean bottom by using a remotely operated vehicle or by simply dropping the seismic nodes in the sea and then let them slowly descend to the ocean bottom. In the latter case the seismic nodes are recovered by a vessel by transmitting a signal that triggers a mechanism in each of the seismic nodes that activates its floating device or releases the seismic node from an anchorage weight such that the seismic node can slowly float up to the sea level by itself. Another way of using these nodes, which has been applied, is to attach the autonomous seismic nodes to a flexible rope, drop the seismic nodes with slack in the rope and then let them descend to the ocean bottom. After the recording is completed the nodes are recovered by winching up the rope.
With the present method, a vessel needs to deploy the seismic nodes, and after the seismic data is recorded retrieve the nodes for further usage. In changing marine environments and due to different weather conditions this cannot always be optimally scheduled. During a typical data acquisition programme, the seismic nodes are placed underwater for a long duration, which could be several days, weeks or months at a time. Throughout the placement, oscillators in the seismic nodes may drift and thereby produce a time error in the sampled seismic data that varies due to for example temperature changes or gravitational forces.
Furthermore, the autonomous seismic nodes will have to operate for these long periods of time without any additional battery charging. The seismic nodes are thus required to be very power efficient.
During offshore oil or gas exploration, the recording precision of the seismic nodes and the parts therein are of vital importance. Due to precision errors in the seismic nodes or any parts thereof, much of the available hydrocarbon and gas may not be mapped with sufficient quality.
One of the major factors affecting recording precision is drift. Drift is the rate at which an oscillator of the seismic node gains or loose frequency in relation to a specified frequency. All oscillators will experience frequency changes though at different rates. Drift in an oscillator causes changes in the frequency of the oscillator of the seismic node, which will results in timing errors. The frequency accuracy of an oscillator is the offset from the specified target frequency. The frequency stability of the oscillator is the spread of the measured oscillator frequency relative its operational frequency during a period of time.
One significant factor affecting the drift and then the recording precision is the temperature coefficient, which may affect how much an oscillator's frequency drift in response to changes in temperature. An oscillator of the seismic node produces a signal at one frequency while on the warmer deck of a source vessel, but may produce a signal at a different frequency when submerged in cold water. The frequency drift of an oscillator will negatively impact recording precision.
In addition to drift caused by temperature, the oscillators may be affected by other environmental variations caused by vibrations, gravity, power supply variations and/or other factors. Crystal aging is another factor that has an impact on the output frequency. Aging in crystal oscillators is caused by a variety of electromechanical mechanisms. Long term stability is usually expressed in parts per million (ppm). A ppm of 15 means that over a 1 ms interval the oscillator period can change by 15 ns. Short term stability is a function of noise signals within the oscillator and represents a phase modulation of the oscillator output. Short term stability can be specified in the time domain as jitter, but depends upon the measurement interval.
A comparison among different type of oscillators is shown in FIG. 10.
Oscillators may also need some time from startup before they reach the necessary stability in their output frequency. According to different accuracy, stability and cost requirements, different types of oscillators are developed. Compensation of the temperature dependence has resulted in oscillators based on different temperature control methods; Temperature Compensation Crystal Oscillators “TCXO”, which uses a temperature compensation circuit; Oven Controlled Crystal Oscillator “OCXO”, which uses an oven to control the crystal temperature.
It is costly and time consuming to place seismic nodes at the ocean bottom, and the weather may restrict time available for data acquisition and this may result in inadequate power to run all the electronics including the oscillators. In addition, many types of oscillators will drift because of temperature variations etc., while others may use so much power that it will limit the number of days the electronics will function. The physical size of existing oscillators and battery power packs required may also be a limitation. If an ocean bottom cable with electric conductors is used, then a power supply onboard the vessel is required, but water might enter electrical terminations and connectors and thus affect the usability of the cable. Also, the cable cannot be too long as this would cause the transmitted voltage to drop to an unacceptable level (the cable may be several kilometers long).
U.S. Pat. No. 4,281,403 discloses a decentralized seismic data recording system wherein individual recording units located remotely from a central station are used for recording of seismic data. The units include a self-contained time counter and means for programming a plurality of recording cycles at desired intervals in synchronization with seismic shots initiated by the central station. A local time counter in each remote unit is compared to the present value in a master clock time counter in the central station. The local accumulated time count as read from the respective remote-unit time counters and the accumulated time count of the master clock are separately recorded on special data files on the archival storage medium in each of the corresponding remote units. The difference in accumulated time between the local clock and the master clock may then be linearly prorated among all of the recorded data files for each of the remote units, thus synchronizing them with the master clock and with each other.
US 2005/0246137 illustrates a method and system for acquiring seismic data without the need for wire line telemetry or radio-telemetry components or radio initiation. A plurality of individual wireless seismic data acquisition units are used wherein the individual data acquisition units may function as data sensor recorders and/or as source-event recorders. Each data acquisition unit records an independent stream of seismic data over time, such as in the form of displacement versus time. The data acquisition units do not require radio contact with other data acquisition units, nor do they require direct synchronization with other receiver units or with a source start time.
US 2009/0080290 discloses a nodal seismic data acquisition system that utilizes an external, common distributed time base for synchronization of the system operation. The system implements a method to correct the local time clock based on intermittent access to the common remote time reference. The method corrects the local time clock via a voltage controlled oscillator to account for environmentally induced timing errors. The invention further provides for a more stable method of correcting drift in the local time clock.
US 2010/0034053 discloses a method for acquiring seismic data by recording seismic data with a plurality of autonomous seismic data acquisition units wherein each acquisition unit comprises a digitally controlled temperature-compensated crystal oscillator. Oscillator-based timing signals are acquired that are associated with the plurality of digitally controlled temperature compensated crystal oscillators and a time correction is determined using the oscillator-based timing signals from the first and second autonomous seismic data acquisition unit.
U.S. Pat. No. 7,254,093 discloses a seismic data collection unit or pod comprising a water tight case. The case houses other components that may include a clock, a power source, a control mechanism and a seismic data recorder. More specifically, Seafloor Seismic Recorders “SSR” units of the Ocean Bottom Seismic “OBS” type generally include one or more geophone and/or hydrophone sensors, a power source, a seismic data recorder, a crystal oscillator clock, a control circuit, and, in instances when gimbaled geophones are used and shear data are recorded, compasses or gimbals.
U.S. Pat. No. 7,558,157 illustrates that in order to reliably and accurately accomplish error-free data from a suite of independent sensors/nodes or an array of sensors, each node includes an atomic clock. In addition, the central data receiver/processor also includes an atomic clock. Each node transmits a time-stamped pseudo-random code. The processor compares the time-stamped pseudo-random code transmitted from nodes with its own internal time-stamped pseudo-random code. By embedding an atomic clock within the processor, data correction and/or calibration is improved in comparison with a conventional GPS receiver not having an internal atomic clock.
U.S. Pat. No. 8,050,140 discloses self-contained ocean bottom pods characterized by low profile casings. A pod may include an inertial navigation system to determine ocean bottom location and a rubidium clock for timing. A clock that is affected by gravitational and temperature effects can cause a frequency shift in the oscillator frequency, thereby resulting in errors in the seismic data. The use of a rubidium clock, which is less susceptible to temperature or gravitational effects or orientation of the unit on the ocean bottom, will result in accurate seismic data recording. The clocks are synchronized with the firing time of the seismic energy source.
Carleton University, systems and Computer Engineering, Technical Report SCE-08-12, Nov. 2008: “Frequency Accuracy & Stability Dependencies of Crystal Oscillators” by: Hui Zhou, Charles Nicholls, Thomas Kunz, Howard Schwartz Cardinal Components Inc. Applications Brief No. A.N. 1006: “Clock Oscillator Stability”
Further, with better and more reliable seismic nodes and the grids or arrays made out of them, the seismic nodes can remain underwater for a longer duration with less maintenance. This will provide more flexibility to operations and also reduce expenses.
The need for lower battery power consumption along with proper dealing with frequency drift remains a major challenge for most seismic node system operations. These issues limit the application of seismic nodes to areas where cable surveys are not an option for operational reasons, for example in the vicinity of platforms or in deep water.
The seismic nodes have been proven to be difficult to operate due to the operational difficulties mentioned above.
The methods of data acquisition described above may not be viable solutions for long duration surveys. The efficiency of these systems is too low and may at times provide inaccurate data.